The present invention relates to a color television signal transmission system, and more particularly to a color television signal transmission system which has resolution and color reproducibility superior to those of the Improved Definition Television system (IDTV) and Extended Definition Television system and which can be realized in a less expensive manner than the High Definition Television system (HDTV). The present invention also relates to a receiver for use in the above mentioned color television signal transmission system.
The color television standard system used in Japan and in the United Stated of America is called the NTSC system. In the NTSC system, the color television signal is a composite signal comprising a brightness signal a chrominance signal, synchronizing signals, and an audio signal, and is compatible with a black and white television signal. As shown in FIGS. 1A and 1B, the frequency bandwidth of the NTSC composite signal, including the audio signal, is limited to 6 MHz, and the frequency bandwidths of the brightness and chrominance signals are limited to 4.25 MHz and 2 MHz, respectively. Therefore, the resolution in the horizontal and vertical directions is restricted to 300 lines. Such a resolution would be satisfactory for domestic purposes, but would not be sufficient for medical and some industrial purposes. Further, due to the fact that the chrominance signal does not have a sufficiently wide bandwidth, the color reproducibility is poor. For medical purposes, microscopic images of blood, bacteria, and cells, and various other kinds of medical images such as images of the bodies of patients during surgery, X-ray images, and ultrasonic images most be transmitted with high resolution and reproducibility. The transmission of such medical images may be required when district medical organizations distributed over the country, or doctors on ships, want to seek suggestions about patients from experienced doctors located at a central medical organization. For such a purpose, vertical and horizontal resolution up to 400 and 750 lines, respectively, must be attained and the color reproducibility should be higher than that of the NTSC system.
Nippon Hoso Kyokai (NHK) in Japan has developed a High Definition Television (HDTV) system called the MUSE system. This HDTV system utilizes the multiple sampling technique and has a very wide frequency bandwidth of 27 MHz. This system can attain the high vertical and horizontal resolution up to 800 lines, and extremely superior color reproducibility can also be achieved. This system can sufficiently satisfy the above mentioned requirement for the medical purpose. However, not only the transmitter, but also the receiver, are very complicated in construction, large in size, and expensive in cost. Further, the resolution of the HDTV system is unnecessarily higher than that required for the above explained purposes. Moreover, in the HDTV system almost none of the equipment in either the transmitter or the receiver of the existing NTSC system could be used.
Some color television signal transmission systems have been developed which are compatible with the NTSC system and have higher resolution than the NTSC system. For instance, the EDTV and IDTV systems have been proposed. In the IDTV system the transmitter transmits the same signal as that of the existing NTSC system, and in the receiver the interlaced signal is converted into a non-interlaced signal by using interpolation, so that the vertical resolution is increased. In the EDTV system, the interlaced signal is converted into a non-interlaced signal as in the IDTV system, and further high frequency components of the brightness signal are interlaced with low frequency components of brightness signal and the chrominance signal so that the horizontal resolution is improved. However, in these known IDTV and EDTV systems, the vertical and horizontal resolution are 450 and 400 lines, respectively, which do not satisfy the necessary resolutions of medical images. Further, the color reproducibility of the IDTV and EDTV systems is too low to reproduce medical images satisfactorily, because the frequency bandwidth of the chrominance signal is restricted.
As explained above, the known color television systems do not satisfy the requirements mentioned above, and thus it would be desirable to develop a new color television signal transmission system which has horizontal and vertical resolution up to 750 and 400 lines, respectively, and color reproducibility superior to that of the NTSC system.